This invention relates to seismic detection of myocardial ischemia secondary to coronary artery disease and related measurement of cardiac performance parameters.
Seismic measurement of chest wall vibrations using an accelerometer sensor mounted on the sternum, generates a waveform signal pattern, an example of which is shown in FIG. 2. The seismocardiographic (SCG) waveform, which is the mechanical equivalent of electrocardiographic (ECG) waveform, may be labeled as shown with point features that correspond to the pumping motion of the heart during systole and diastole. The SCG waveform contains information that allows for the noninvasive estimation of various cardiac performance parameters such as ejection fraction (percentage of heart volume pumped on each stroke) and cardiac output (flow in liters per minute). This same SCG waveform also contains information on abnormal heart state conditions such as myocardial ischemia (or angina) in which the heart becomes starved for oxygen because of inadequate blood flow. Myocardial ischemia is a precursor of myocardial infarction (heart attack) in which part of the heart tissue dies because of prolonged oxygen starvation.
SCG technology presents the opportunity to noninvasively measure cardiac performance and detect life-threatening abnormal heart conditions with a single rapid measurement using an operator with minimal medical skills. Current methods of measuring cardiac performance are either extremely invasive (and costly) or noninvasive imaging approaches that require time-consuming measurements and highly skilled operators. Examples of the former invasive techniques are angiographic ventriculography, nuclear ventriculography and right heart catheterization. An example of the latter is echocardiography which provides ultrasonic dynamic images of the heart requiring expert level interpretation. In contrast, with the present invention using SCG, rapid estimation of cardiac performance parameters can be performed rapidly and noninvasively with no requirement for skilled interpretation. A simple readout may provide ejection fraction and cardiac output values in less than one minute of time.
The same situation exists in the detection of myocardial ischemia. Current methods emphasize the use of resting ECG or stress ECG neither of which is very sensitive for myocardial ischemia detection. More sensitive and specific methods such as nuclear (Thallium) imaging are time-consuming, invasive, very costly and require expert interpretation. Again, SCG detection of ischemia can be accomplished quickly, with high sensitivity and specificity and with a simple readout rather than expert interpretation, with the present invention.
Previous attempts at harnessing the power of seismocardiographic waveforms have been handicapped by the lack of advanced instrumentation, signal processing and pattern recognition techniques. Seismocardiography represents the latest development in a general category known as displacement cardiography. This field involves low frequency acoustic measurements in the 0-50 Hz range. The portion of this spectrum below 25 Hz is often called infrasound since it is below the human spectral hearing range. Earlier versions of displacement cardiography were designated as ballistocardiography, kinetocardiography and apexcardiography (Salerno 90). Out of all of these earlier developments, only apexcardiography survives as an auxiliary measurement in occasional use today. The field, however, has an interesting history, and a form of displacement cardiography was employed in monitoring cosmonauts earlier in the Soviet space program. The terms seismocardiography and SCG were coined in 1959 by a Russian physician, B. S. Bozhenko. During the 1960's and 1970's, Dr. Bozhenko and his associates conducted research using crude accelerometers and recording equipment. The technology, however, never received widespread acceptance even in the Soviet Union (Gelis 61). Because ofthe limited scientific contact between the Soviet Union and West during the Cold War, the Soviet's work on this technology was not known in the West until the late 80's.
SCG technology development in the U.S. has also been limited by the need for expert interpretation. U.S. Pat. No. 4,989,611, which issued Feb. 5, 1991, and U.S. Pat. No. 5,159,932, which issued Nov. 3, 1992, teach the use of SCG technology in cardiac performance evaluation. The '611 patent relates primarily to measurement and display of an averaged SCG waveform. The '611 patent also describes Fourier transform-based processing and a display in the frequency domain. The attending physician must then interpret the results. The '932 patent similarly provides graphic information for detection of myocardial ischemia. Again, the information is presented graphically and must be interpreted by the physician.
Similar experience has been reported in SCG estimation of cardiac performance parameters. Although no known patent literature addresses these functions, informal communications with John Zanetti, a co-inventor of the above two patent disclosures, indicate a preference for using the SCG waveform to estimate various time intervals in the systolic portion of the SCG waveform. These systolic time intervals allow for the use of established formulas to estimate fraction, stroke volume and cardiac output [Harley 69] [Garrard 70]. This approach has two significant shortcomings:
1. It is extremely difficult to automate the identification of marked points on the SCG waveform. Algorithms developed to date have a high error rate due to the wide variation in SCG waveform characteristics. PA1 2. Systolic time interval formulas, while reasonably accurate for most patients, perform very poorly for patients with abnormal waveforms caused by valvular and other heart diseases.
This patent application addresses previous difficulties in automated SCG waveform interpretation for both cardiac performance estimation and myocardial ischemia detection by converting the SCG waveform into the parameters of a mathematical model that accurately represents the SCG waveform and allows for a rapid automated cardiac performance estimation and myocardial ischemia detection in a robust manner without the need of physician or other expert interpretation.
U.S. Pat. Nos. 4,989,611 and 5,159,932, as previously discussed, use SCG technology with the information presented graphically that must be interpreted by the physician.
The SCG waveform, the mechanical equivalent of the well-known electrical ECG waveform, provides information on the functioning of the heart as a pump as opposed to the electrical command signals of the ECG. The SCG waveform records what the heart actually does while the ECG records the signals that direct what the heart is supposed to do.
The present inventor recognized that the waveform structure provided by SCG technology permits formulation of model-based algorithms for cardiac performance measurement and myocardial ischemia detection and therefore avoids the necessity of the visual interpretation of the results by a highly specialized physician and providing, for example, a numerical value for practical clinical application.
Further, although applicable to estimating various diagnostic cardiac performance parameters, SCG has also been recognized as a useful development for detection of myocardial ischemia as noted in the '932 patent.
However, the inventor has recognized the present state of the art includes significant limitations for clinical application. The prior art presentation of visual and therefore subjective information rather than a more definitive objective information has prevented widespread practical application for this technology and particularly non-existent clinical application. Given the continuing emphasis on cost and productivity in the art of cardiac evaluation, the medical community is continuously interested in instrumentation which produce a number relative to a reference to produce a response as to the state of the patient.